Yes, it is true, you got a housing with your GoPro, but this housing is different. The case that comes with your gopro is pretty versatile. It allows you to attach multiple styles of back doors in order to achieve better audio, water proof the camera or even allow for accessories like the extra battery or LCD. What it doesn't allow you to do allow access to camera ports. Why would you want to do that? Well, to be honest, not everyone will need to. However if you want to harness the power of the GoPro by utilizing the Live-Feed video, Charging or Enhanced audio features, then you will be one of the people who will want this feature.
The Skeleton housing is not waterproof, but it provides access for those of us who like to utilize all the advanced features of the camera. If you just want an extra housing, that is fine too, just remember, this one is for use in the dry.